1. Field of the Invention
The present invention relates to hydraulic couplings for use with vehicle drivetrains within a housing thereof containing hydraulic fluid to rotatably couple a pair of rotary members about a rotational axis. More specifically, the present invention relates to such a hydraulic coupling that is electronically controlled.
2. Description of the Related Art
Hydraulic couplings are known in the related art for use with vehicle drivetrains within a housing thereof containing hydraulic fluid to rotatably couple a pair of rotary members about a rotational axis. Hydraulic couplings of this type typically include a casing having a generally hollow construction that is rotatable about an axis extending through the housing. A hydraulic pump is operatively supported within the casing about the rotational axis. One type of hydraulic pump that may be employed for this purpose includes a toothed impeller rotatably connected to one of the rotary members and having external teeth. The hydraulic pump also includes an outer pump element mounted to the casing for rotation eccentrically with respect to the toothed impeller. The outer pump element includes internal teeth of a number one more than the impeller teeth and in meshing relationship therewith to provide a pumping action upon relative rotation between the casing and the toothed impeller as the pair of rotary members rotate relative to each other. An inlet port is provided to the pump through which hydraulic fluid is delivered from a sump formed in the housing to the casing via the hydraulic pump. The hydraulic coupling further includes at least one clutch assembly that is employed to couple the rotary members. The clutch assembly typically includes a piston that is movably supported within a piston chamber formed within the casing. Pressurized, hydraulic fluid is delivered to the piston chamber and acts against the piston to engage a clutch pack thereby coupling the two rotary members to each other. To this end, the casing may include a transfer port through which pressurized hydraulic fluid is pumped from the hydraulic pump to the piston chamber. In addition, the casing may also include at least one outlet port through which pressurized hydraulic fluid flows from the piston chamber and is ultimately delivered back to the sump in the housing. Hydraulic couplings of the type described above and known in the related art may further include a self-actuating pressure or temperature responsive control valve that automatically opens and closes the outlet port in response to the pressure and/or temperature of the hydraulic fluid in the piston chamber thereby engaging or disengaging the clutch and couple or decouple the pair of rotary members, respectively.
Examples of hydraulic couplings of the type described above for use with front and rear differentials as well as transfer cases can be found in U.S. Pat. Nos. 5,310,388 issued to Okcuoglu et al.; 5,536,215 issued to Schaffer et al.; 5,595,214 issued to Schaffer et al.; 5,735,764 issued to Schaffer et al.; and 5,888,163 issued to Schaffer et al. Each of these patents are assigned to the assignee of the present invention. Furthermore, the disclosures of each of these patents are expressly incorporated herein by reference.
While the hydraulic couplings having self-actuating pressure or temperature esponsive outlet port control valves of the type referenced above have performed well in the past, there remains a need in the art for a hydraulic coupling which may be selectively actuated in an xe2x80x9con demandxe2x80x9d mode to couple the rotating members as a function of parameters which may be independent of the pressure acting against the piston in the piston chamber. For example, in certain circumstances, vehicle driving conditions may dictate that the rotary members are preferably coupled at all times and without regard to the differential rotation of the axle half-shafts. Such circumstances may include vehicles that are driving in extreme off road conditions where it is desirable to have the axle half-shafts locked together.
On the other hand, circumstances may dictate that the rotating axle half-shafts are uncoupled without regard to any differential rotation of the axle half-shafts. Such circumstances may include vehicles that are operating in extreme cold weather conditions. Still further, it may also be desirable to regulate the amount of slip of the hydraulic coupling as a function of vehicle drivetrain parameters that are independent of the pressure generated by the hydraulic pump due to the differential rotation of the rotating axle half-shafts. Such parameters may include one or more of the yaw rate, vehicle speed, differential fluid temperature, differential axle speeds between the axle half-shafts or torque, etc.
Although there are a number of driving conditions that may benefit from the selective xe2x80x9con demandxe2x80x9d and/or xe2x80x9cautomatic controlxe2x80x9d of the limited slip function of the hydraulic coupling, another example may include the use of a mini spare tire for emergency purposes. Since such spare tires have a relatively small diameter, continuous operation of the vehicle with traction wheels of a different effective diameter results in undesirable speed differences over extended periods of time. However, those having ordinary skill in the art will appreciate that there are many possible driving circumstances that would be improved by a hydraulic coupling having a limited slip capabilities that are actuable independent of the pressure generated by the hydraulic pump.
Thus, there remains a need in the art for a hydraulic coupling for use with vehicle drivetrains wherein the couplings have xe2x80x9con demandxe2x80x9d, xe2x80x9csystem offxe2x80x9d and xe2x80x9cautomaticxe2x80x9d limited slip operational modes.
The present invention overcomes the deficiencies in the related art in a hydraulic coupling for use in a vehicle drivetrain including a housing and a pair of rotary members projecting therefrom. The hydraulic coupling includes a hydraulic circuit defining a low-pressure portion wherein the hydraulic fluid is at a first, lower pressure and a high-pressure portion wherein the hydraulic fluid is in a second, elevated pressure relative to the first pressure. A hydraulic pump is operatively supported within the housing and located within the path of the hydraulic circuit between the low pressure and high-pressure portions. The hydraulic pump is operable to pump hydraulic fluid from the low-pressure portion to the high-pressure portion of the hydraulic circuit. In addition, a clutch is supported within the housing and located within the high-pressure portion of the hydraulic circuit. The clutch is actuable by hydraulic fluid that is pumped by the hydraulic pump at the second, elevated pressure to couple the pair of rotary members together. Furthermore, the hydraulic coupling of the present invention includes a control valve that is operable in response to a signal from a controller to selectively control the actuation of the clutch independent of the fluid pressure in the high-pressure portion of the hydraulic circuit.
Accordingly, one advantage of the hydraulic coupling of the present invention is its ability to operate in a xe2x80x9con demandxe2x80x9d mode, a selective or xe2x80x9cautomatic controlxe2x80x9d mode or a xe2x80x9csystem offxe2x80x9d mode wherein each of these operational modes are transparent to the vehicle operator. Furthermore, because the control valve is electrically actuated, the hydraulic coupling of the present invention permits operation that approaches open differential with minimal cross-axle torque transfer. Thus, it may be operated independently for maximum torque transfer, or in conjunction with anti-lock braking systems (ABS) based on traction control. Alternatively, the ABS traction control can be operated independently of the hydraulic coupling of the present invention. Thus, the present invention permits a wide range of torque transfer capability for maximum traction control.
Another advantage of the present invention is that the control valve provides the capability to operate the vehicle with large relative rotational speed differences between the axle half-shafts where desired thereby permitting use of mini-spare tires. In such circumstances, the limited slip differential capabilities are operated in a xe2x80x9csystem offxe2x80x9d mode up to a limiting maximum speed.
Still another advantage of the hydraulic coupling of the present invention is that torque transfer across the axle half-shafts can be activated or deactivated at any time during vehicle operation without damage to differential components due, for example, to torque spikes that result in damage to gears or the clutch.
Still another advantage of the hydraulic coupling of the present invention is that it allows for proportional differential torque transfer in response to an input signal receiving various vehicle operational and drivetrain data such as axle half-shaft speed differences, shaft torque differences, as well as input regarding the relative pressures in the hydraulic circuit. Accordingly, the hydraulic coupling of the present invention includes a control circuit that facilitates this operation.
The hydraulic coupling of the present invention may be employed as a part of front or rear differentials, transfer cases or hydraulic couplings employing one or more hydraulic pumps and clutch systems without differential gearing that are used to control torque transfer between front and rear axles in all wheel drive vehicles.